The DEFAULT constraint provides a default value to a column when the INSERT INTO statement does not provide a specific value.
For example, the following SQL creates a new table called CUSTOMERS and adds five columns. Here, SALARY column is set to 5000.00 by default, so in case INSERT INTO statement does not provide a value for this column, then by default this column would be set to 5000.00.
CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2) DEFAULT 5000.00, PRIMARY KEY (ID) );
If CUSTOMERS table has already been created, then to add a DFAULT constraint to SALARY column, you would write a statement similar to the following:
ALTER TABLE CUSTOMERS MODIFY SALARY DECIMAL (18, 2) DEFAULT 5000.00;
To drop a DEFAULT constraint, use the following SQL:
ALTER TABLE CUSTOMERS ALTER COLUMN SALARY DROP DEFAULT;